I propose to study the relationship between the binding of thyroid hormones to nuclear and cytoplasmic binding proteins and the change in gene transcription which are induced by the hormone, using the tadpole as a model system. The binding will be measured in two tissues, the liver and intestine, which undergo vastly different alterations during metamorphosis. Conditions will be established which will permit the measurement of sites which are free to bind added thyroid hormone and total binding sites (free sites plus those which are filled with thyroid hormone in vivo) as well as the dissociation constant for the binding of hormome. The number of sites will then be measured throughout metamorphosis to determine if there is synthesis of new binding proteins as the circulating levels of the hormone increase. The cytoplasmic and nuclear binding proteins from the liver and intestine will be analyzed by sucrose gradient centrifugation, Sephadex gel chromatography, and disc gel electrophoresis to determine if there is any physical relationship between the cytoplasmic and nuclear binding proteins as well as the relationship between the binding proteins of the two tissues. The possibility that a thyroid hormone-cytoplasmic binding protein complex serves to transfer thyroid hormones to the nucleus will be investigated using in vivo and in vitro techniques. These studies will serve to advance our understanding of the mechanism by which thyroid hormones control the genetic expression of the cell.